The present invention is generally related to subsea wellhead systems, and more particularly to a casing hanger for suspending an intermediate casing string from a subsea wellhead housing, and a separately installable landing shoulder insert or support member for supporting progressively smaller sized casing strings within the intermediate string and for sealingly isolating the intermediate casing string hanger from internal fluid pressure in the subsea wellhead housing.
In the past, subsea wellhead systems have been known and used in the drilling of underwater wells for the production of oil and gas which utilize a separately installable landing shoulder insert or support member for multiple concentric casing strings and hangers in order to allow full bore access, without underreaming, below the wellhead housing for a standard 171/2 inch drill bit prior to installation of the multiple concentric casing strings. The multiple concentric casing strings, or surface casing, may be, for example, 133/8 inch, 95/8 inch, and 7-inch strings, all supported on the landing shoulder insert attached to the wellhead housing and concentrically disposed within a conductor casing string, typically a 20-inch string welded to the bottom of the wellhead housing. It has also been proposed that such a separately installable landing shoulder serve as a hanger for an intermediate casing string, such as a 16-inch string, between the 20-inch conductor casing and the 133/8 inch casing. Examples of subsea wellhead systems such as the foregoing are disclosed in U.S. Pat. No. 4,615,544, issued Oct. 7, 1986, which is incorporated in its entirety herein by reference.
At times, it may be necessary or desirable, due to formation conditions or other factors, to install and cement an intermediate casing, such as a 17-inch or a 16-inch casing, for example, in the well prior to running the separately installable landing shoulder into the wellhead. In order to do so, however, according to past practice the wellhead would have to have an integral landing shoulder which would be large enough to support the weight of the intermediate casing hanger and string, as well as pressure end loads resulting from internal fluid pressure in the wellhead housing. The potential pressure end loads might be, for example, as much as eight times the casing weight. Such an integral shoulder would have to extend far enough into the wellhead bore to provide sufficient bearing area for the intermediate casing weight and pressure end load, so as to restrict the bore and prevent passage therethrough of the standard 171/2. inch drill bit. In that event, underreaming would be required below the wellhead housing, thereby negating one of the primary advantages of the separately installable landing shoulder insert.